To realize a low-altitude (especially near-ground) autonomous flight of an unmanned aerial vehicle (UAV for short), both an altitude of the UAV and a height of the UAV relative to the ground are needed to know. The height of the UAV relative to the ground may be obtained by a sonar ranging, a laser ranging, a microwave radar ranging and a machine vision measuring, etc.
The sonar ranging is not susceptible to light. However, the sonar ranging used in the related art is in an environment totally different from the UAV, for example, a high-frequency vibration of a fuselage of the UAV caused by high speed rotation of a screw propeller, airflow disturbance caused by rotation of the screw propeller, rapid and repeated tilt changes of the attitude of the UAV during flight, unstable power supply caused by high speed rotation of the screw propeller, and the complicated ground environment during an air-to-ground ranging process of the UAV. These complex situations may bring serious noise into a UAV airborne sonar ranging.